Heap leach mining is a common process for extracting metals, such as Gold, Copper and Uranium, from ore. The mined metal may be a base, precious, and/or strategic metal. In basic terms, heap leach mining is the process of mining ore that contains metal, placing the ore on a pad (having a collection system) in a stack or heap, applying solvents to the top of the heap, and collecting the solvent (which includes dissolved metal leached from the ore, known as pregnant leaching solution or “PLS”) via the collection system at the bottom of the heap. The metal is then extracted from the recovered PLS. In some operations, after the metal is recovered, the solvent (now considered “barren”) may be re-used.
The pad may be impermeable so that the solution does not soak into the ground. The pad may be placed on slightly sloping ground to facilitate recovery of the PLS after it has passed through the heap and accumulates at the lowest part of the sloping pad. If this method is used, the slope should not be such as to make the heap unstable.
The ore may or may not be crushed before placement in the heap. Some crushing processes include both primary and secondary crushing to efficiently obtain a preferred ore size. Uncrushed ore placed in heaps is commonly described as “run of mine” or “ROM” and the heaps such ore is placed on are commonly known as a “stockpile.” Unless otherwise specified, the term heap leach as used herein includes ROM operations as well as crushed ore operations, which are more commonly thought of as “heap leach,” and the term “heap” includes ROM stockpiles. The solvent is often applied using sprinklers or drip systems for efficient distribution without excessive evaporation.
A stack of the ore above the pad is called a “lift.” In some heap leach operations, ore is laid down having a particular lift height, and solvent is applied and recovered for some time. Then additional ore is placed on top of the first lift and the process is repeated. There may be several stacks of ore on one pad, as the process continues. These are called “multi-lift heaps.” If the leached ore is removed before another lift is put on the pad, the operation may be called an “on/off” pad. Single lift operations, where there is only one lift of ore, are also used.
The leaching process (that is applying the solvent to the heap and collecting enriched solvent at the bottom of the heap at the pad) is an ongoing process, with more solvent being applied to the top of the heap until as much metal as is practicable has been recovered. This process may take months and may exceed one or more years, especially in ROM operations. At any time after the process begins, a certain amount of solution is in the heap along with the metal it contains, so that there may be a certain percentage of the value of the metal from the mine that cannot be obtained until all solution is recovered and the operation is shut down.
In addition (or as an alternative) to conventional solvents, bacteria may be applied in solution in some heap leach operations to ingest and transport the metal. The bacteria may have certain needs in order for a bioleaching process to work efficiently, such as appropriate temperatures and adequate levels of iron and oxygen in the bacterial solution.
While one may consider the step of mining the ore as part of the heap leach process or as a process preliminary to the heap leach process, heap leach mining operations may involve ongoing interaction between excavation of the ore and the heap operation itself. The leaching process is highly dependent upon ore properties, such as grade, mineralogy and particle size of the ore. These ore properties may vary across the area of extraction and/or as a result of the operating plan.
Mining operations are typically pre-planned with the aid of a block model of the mine from which the ore is extracted, the block model having estimates of ore grade and other rock properties throughout the block of ore to be extracted. Samples may be collected and analyzed from the mine to obtain these estimates. Using the block model, estimates are made of the ore grade, mineralogy, and particle sizes which will be available for leaching as the excavation and heap leach process progress. This prediction is used to devise heap building and leaching operating plans. Simplified hydrological and empirical metallurgical models are used to develop pre-operational estimates of the leaching process. However, deviations from the predicted ore grade, mineralogy, and particle size pre-mining plan result in deviations from the heap caused by pre-mining estimates, which in turn diminish the validity of leaching predictions. These factors result in potential operational inefficiency and difficulty in making accurate estimates of cash flow.
The heap leach process can take up to several months per pad and, therefore, monitoring how efficiently the metals are leached an be a challenge. Many factors contribute to the efficiency of a heap leach operation, including temperature of the heap as well as the method of application of the solvent to the heap.